1. Technical Field
The present invention relates to a thin-film transistor, an electronic circuit, a display unit, and an electronic device.
2. Related Art
Recently, organic thin-film field-effect transistors including an organic semiconductor material have attracted attention as a device that can replace thin-film field-effect transistors including an inorganic material such as silicon. In such organic thin-film transistors, a semiconductor layer can be formed by a liquid phase process that does not require a high temperature or a high vacuum. in addition, the organic thin-film transistors are advantageous in that, for example, they can be thinner and lighter than the inorganic thin-film transistors, they have flexibility, and the cost of materials is low. In light of these characteristics, the organic thin-film transistors are expected to be used as a switching element of flexible displays and so forth.
A top-gate structure and a bottom-gate structure have been proposed as such thin-film transistors In the top-gate structure, a source electrode and a drain electrode are provided on a substrate in parallel, an organic semiconductor layer is provided between the source electrode and the drain electrode, and a gate insulating layer and a gate electrode are provided thereon in that order. In the bottom-gate structure, a gate electrode and a gate insulating layer are provided on a substrate in that order, a source electrode and a drain electrode are provided on the gate insulating layer in parallel, and an organic semiconductor layer is provided between the source electrode and the drain electrode.
In the organic thin-film transistors, preferably, components other than the organic semiconductor layer can also be formed by a liquid phase process. Examples of a gate insulating layer formed by a liquid phase process include a SiO2 film prepared by applying a solution of polysilazane and heat-treating the solution at a temperature of about 400° C., and a spin-on-glass (SOG) film prepared by applying a solution of an organic silicon compound and heat-treating the solution at a temperature of about 400° C.
However, heat treatment at about 400° C. is required in order to form the above gate insulating layer, resulting in a problem of thermal degradation of the organic semiconductor layer and a plastic substrate by heating. Another example of an insulating film formed by a liquid phase process is an organic polymer film prepared by applying a solution of an organic polymeric material or a precursor thereof and performing an after-treatment. This after-treatment is, for example, a formation treatment of a cross-linked structure or a polymerization treatment of the precursor, and performed by heating at a temperature significantly lower than 400° C., for example, at about 100° C. Accordingly, when an organic polymer film is used as a gate insulating film, the above-described problem of thermal degradation of the organic semiconductor layer etc. can be prevented. However, such an organic polymer film has a low withstand voltage characteristic and easily causes dielectric breakdown when the gate voltage increases (for example, see “Plastic Data Book” published by Kogyo Chosakai Publishing, Inc. and written and edited by Asahi Kasei Amidas Co., Ltd. and the editorial department of “Plastics”, Dec. 1, 1999, pp. 187-188).